INFRARED COMMUNICATION - some problems with led

[Bukitoo]

INFRARED COMMUNICATION

Hi !!

I am developing a IR link, and y need pulses with 1us of rise time. I have problems with the LED specifications.
FOr example: I have LED with a switching time of 800 ns, but when I try to make it work, It works up to 3 kHz. !!!

I don't know what to do. It 's the driver ?

 

[flatulent]

Re: INFRARED COMMUNICATION

How did you determine the frequency? Did you monitor the voltage across the diode or did you use an IR detector?

Here are some tips.

The driver should have a high current capacity. Several buffer inverters in parallel. Put a resistor between them and the LED to set the current. Put a capacitor across the resistor to make the initial charging and discharging of the diode junction fast. Start with a low value of 10 pF and raise it by steps of 2X until you get the voltage across the diode to be a good rectangle with fast edges.

 

[Bukitoo]

Re: INFRARED COMMUNICATION

I used a photo detector. With 3kHz periodic pulse waveform as source of the transmitter, the received waveform was unacceptable form me. THat was what I mean.

Another thing that I don't understand very well is the calculation of the max current that the LED could manage. In my design I need pulses of 10 us with a max dutycycle of 1/2.

Thanks !

 

[flatulent]

Re: INFRARED COMMUNICATION

The LED data sheet should list maximum current. 10 mA is usually well within most diode ratings. That is why LED drivers always have series resistors or transistor current sources to limit the current. Otherwise you will overheat the diode and damage it.

Another thing is overdriving the detector. Put more distance between them and see if things improve.

 

[Bukitoo]

Re: INFRARED COMMUNICATION

Yes. I have a led with 100mA maximum dc current, but I want to know about how to calculate the peak current with my requirements.

Thanks!

 

[flatulent]

duty factor

There should be a graph of maximum current vs duty factor and pulse width. Your 100 mA and the duty factored should be safe.

 

[Bukitoo]

Re: INFRARED COMMUNICATION

Yeah !!! I've found it !
Thanks !!!